Thermal power generation plays a major role in our country power generation industry, the installed capacity of the thermal power has been higher than 70%. Although advantaged in low cost to control pollution, high fuel applicability, wide load regulation range and so on, circulating fluidized bed combustion technology causes corrosion to a device such as a boiler heating surface and fouling and slagging when burning a high-alkalinity coal as the alkali compounds in the coal, after volatilizing during the combustion process, are likely to condense on a boiler heating surface to form a sintered or adhered ash deposit. Fouling and slagging will reduce the heat transfer efficiency of a boiler, lower the output of the boiler and severely impair the operation safety of a device.
To avoid the various problems caused by fouling and slagging, a lot of research has been made on the mechanism of fouling and slagging by scholars at home and abroad, the result shows that fouling and slagging is a complicated physical and chemical reaction process and that the slagging in a boiler is both a complicated physical and chemical process and a dynamic process and is related to both fuel characteristics and the structure and the running conditions of the boiler. A plurality of slagging determination indexes have been proposed by the scholars which confront many limitations in the actual application and therefore only serve for a preliminary determination but cannot fundamentally eliminate the damages caused by contamination to a boiler. During the running process of a power plant, the combustion of pulverized coal generates high-temperature smoke and ash, for a high-alkalinity coal, the alkali metals contained in the high-alkalinity coal volatilize in a gas at a high temperature, the gas flows to a subsequent convective heat exchange surface with the high-temperature smoke, and after the gas contacts with the convective heat exchange surface relatively low in temperature, the alkali metals deposit on the surface of a convective heat exchanger and with a relatively high viscosity, absorb fly ash to generate contamination to the heat-absorbing surface. For a high-alkalinity coal, research shows that due to the volatilization of the alkali metal elements in the high-alkalinity coal, the adhesive deposit basically formed by alkali metal salts, calcium sulfate or the eutectic of sodium, potassium, calcium and sulfates exists mainly in the form NaCl or Na2SO4. The continuous absorption of the deposit to fly ash causes varying degrees of contamination to the convective heat-absorbing surface, moreover, the contaminants which cannot be removed using a soot blower reduce the heat transfer capability of the heat-absorbing surface, increase the temperature of the smoke discharged from the boiler and finally greatly reduce the output of the furnace of the boiler to shut down the boiler.
Thus, if the proportion of the alkali metal compounds in the smoke can be reduced, then the contamination to the convective heat-absorbing surface of the boiler can be fundamentally solved or relieved.
At present, there is a domestic lack of the engineering operation experience on the use of the combustion of a high-alkalinity coal, only several power plants in Xinjiang are studying the problem of the contamination caused by the combustion of a high-alkalinity coal but have not developed any effective high-alkalinity coal utilization method. Although a method is available by means of which the slagging of a boiler is relieved by controlling the temperature and the combustion in a furnace through the optimization of the combustion mode of the boiler, the method, which cannot be operated conveniently in the actual application, is not popularized. As to a method of relieving the contamination to a boiler through non-local coal blended combustion, the proportion of the high-alkalinity coal blended for combustion should be below 30% when ZhunDong coal is blended with other coals for combustion. When the proportion of the high-alkalinity coal blended for combustion is increased, the contamination caused by an ash deposit to the convective heat-absorbing surface of a boiler is severe; meanwhile, alkali metals cause serious corrosion to the material of the body of the boiler, thus making it difficult to design and operate a circulating fluidized bed's boiler. As high-alkalinity coals are mainly used by electric power stations near coal-mines in Xinjiang, a high amount of non-local coals is needed for blended combustion, which not only greatly limits the amount of the used ZhunDong coal but also requires the purchasing of high-quality fire coals from other places, as a result, the power generation cost of power generation enterprises is increased. Consequently, it is difficult to exploit ZhunDong coal fields and construct power source bases, and the exploitation of the advantages of Zhundong coal to the full is hindered. For this reason, it is urgently needed to solve the problem of the contamination caused to a convective heat-absorbing surface when a boiler merely burns high-alkalinity coals.